At present, due to its great potential value in the public safety field and the ordinary civil communication field, the Device to Device (D2D) communication technology has been accepted by the 3GPP standards, and has realized the standardization of a part of functions in the 3GPP Rel-12 and Rel-13, including the mutual discovery of D2D terminals in a In Coverage (IC) scenario, a Partial Coverage (PC) scenario and an Out of Coverage (OC) scenario, and the broadcast communications between D2D terminals.
At present, two D2D broadcast communication modes (Mode 1 and Mode 2 for short) have been defined in the 3GPP Rel-12. Wherein, Mode 1 requires that a UE transmitting D2D broadcast communication must be a UE in coverage of a cellular network (ICUE). The UE acquires configuration information of a D2D Scheduling Assignment (SA) resource pool of Mode 1 by receiving a system broadcast signaling transmitted by an eNB, the configuration information containing the cycle of the SA and the position of a subframe for transmitting the SA in each cycle. When a UE supporting the Mode 1 broadcast communication has data, the UE applies for a dedicated Mode 1 communication resource from the eNB by a particular Buffer Status Report (BSR). Subsequently, the UE detects a D2D grant of the eNB before each SA cycle, and acquires the position of a resource for transmitting the SA and data in this SA cycle. In Mode 1, the collision of resources between different UEs can be avoided through the centralized control by the eNB.
A UE transmitting a D2D broadcast communication by Mode 2 can be an ICUE, or a UE out of coverage of the cellular network (OCUE). The ICUE acquires an SA resource pool for the Mode 2 and a corresponding data resource pool configuration by receiving a system broadcast signaling from the eNB, and then randomly selects the transmitting resource for the SA and corresponding data in each SA cycle; the OCUE determines an SA resource pool for the Mode 2 and a corresponding data resource pool configuration through preconfiguration information, the way of selecting a resource is the same as the ICUE. In a PC scenario, the Mode 2 resource pool configuration preconfigured by the OCUE is related to a carrier frequency of a cell of an ICUE participating in the D2D broadcast communication, a system bandwidth and/or a TDD configuration.
Since the standardized D2D communication in the 3GPP Rel-12/13 is mainly specific to low-speed terminals, as well as services with lower requirements on the time delay sensitivity and receiving reliability, the realized D2D functions are far unable to meet the user demands. Accordingly, in the subsequent 3GPP versions, further enhancing the functional framework of D2D has become a broad consensus for various communication terminal manufacturers and communication network equipment manufacturers nowadays. Wherein, based on the current D2D broadcast communication mechanism, supporting low-delay and high-reliability direct communications between high-speed equipments, between a high-speed equipment and a low-speed equipment and between a high-speed equipment and a static equipment (i.e., V2X (Vehicle to Vehicle/Pedestrian/Infrastructure/Network) is one of functions to be standardized preferentially. Compared with the existing D2D communication environment, in a V2X communication environment, the average movement speed of a transceiver is far higher than that in the existing D2D communication environment. In addition, the density of transmitting users is considered to be low (there are three transmitting users in one cell) in the D2D communication mechanism design process, and in contrast, the density of transmitting equipments in the V2X even in a typical traffic environment is higher than that in the D2D. If a special traffic environment (e.g., a traffic jam situation) is taken into consideration, a difference in the density of transmitting equipments will be larger. In accordance with the conclusions from the 3GPP at present, the V2X communication still follows the Mode 2 of the D2D broadcast communication. If the density of transmitting terminals within a certain region is very high, the use of the Mode 2 communication mode will inevitably cause serious resource collision and in-band emission and further influence the communication reliability.
In addition, compared with the D2D communication, the V2X communication has higher requirements on the time delay and reliability. At present, the 3GPP has clearly stipulated that the minimum time delay for the V2X is 20 ms, and has also provided requirements on high reliability. However, in the existing D2D design process, the requirements on the two aspects have not yet been used as main consideration factors. Therefore, the existing D2D communication cannot meet the requirements on both the time delay and the reliability in the V2X communication.